Geochemical characterization of deep-sea sediments on the Azores Plateau – From diagenesis to hydrothermal activity

• Published in: Marine Geology
• Main Authors: Schmidt, Christopher, Hensen, Christian, Hübscher, C., Wallmann, Klaus, Liebetrau, Volker, Schmidt, Mark, Kutterolf, Steffen, Hansteen, Thor H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2020
• Subjects: Mid-Atlantic Ridge; North Atlantic
• Online Access: https://oceanrep.geomar.de/id/eprint/50224/; https://oceanrep.geomar.de/id/eprint/50224/1/Schmidt_et_al_Azores.pdf; https://oceanrep.geomar.de/id/eprint/50224/7/1-s2.0-S0025322720301791-mmc1.docx; https://doi.org/10.1016/j.margeo.2020.106291

Highlights • Overview on geochemical composition of pore water and solid phase of sediments on the Azores Plateau. • Evidence for deep marine hydrothermal activity on the Azores Plateau. • Pore water data suggest ongoing anaerobic oxidation of methane and carbonate recrystallization. The Azores Plateau is an active magmatic region in the Central North Atlantic Ocean. In this study, we present a comprehensive data set of major element compositions and 87Sr/86Sr ratios of pore waters from surface sediments (0–9 mbsf) of the Azores Plateau. Based on distinct geochemical signatures we can separate normal marine from hydrothermally affected sediments. Normal marine sediments can further be differentiated by their ash content. Pore waters of ash rich gravity cores (GCs) do not show any deviations from seawater values except of a minor increase in Sr. In contrast, ash poor GCs generally show a trend for decreasing Ca with increasing depth, accompanied by a minor SO4 decrease and a more pronounced Sr increase. We suggest that these deviations are caused by processes such as anaerobic oxidation of methane and carbonate recrystallization. At four additional sample locations we observed a decrease in Mg and SO4 accompanied by a Ca increase in the pore waters, a pattern typical for hydrothermal fluids. The existence of hydrothermal systems in this region are corroborated by multi-channel seismic data, suggesting that sill or dyke intrusions are present in the subsurface close to the core locations. Overall, our observations offer preliminary indications for the existence of submarine hydrothermal systems on the Azores Plateau away from the Mid- Atlantic Ridge.